WATERVILLE — The Planning Board on March 6 will consider preliminary and final plans for a proposed $25 million Colby College downtown student residential complex on part of The Concourse that will include a retail use on the ground floor. The meeting is scheduled for 7 p.m. in the council chamber at The Center at 93 Main St. Colby officials have said they want to break ground on the project this spring and have the five-story building, which would consist of about 100,000 square feet, ready for occupancy in the fall of 2018. The 200 students living in the building would be part of a special curriculum involving civic engagement, and the building design will reflect that college-community interaction.